The term SiAlON, or silicon aluminium oxynitride, encompasses a family of compounds or phases comprised of the elements: silicon, aluminium, oxygen and nitrogen. Each phase is described by a composition range for which that particular structure is stable. .beta.'-phase SiAlON (.beta.'SiAlON) is stable over the composition range: Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z=0 to 4.2. This includes silicon nitride (.beta.Si.sub.3 N.sub.4) as the z=0 end member. .beta.'SiAlON has the same structure as silicon nitride (.beta.Si.sub.3 N.sub.4), and can be regarded as a solid solution formed by substituting equal amounts of aluminium and oxygen for silicon and nitrogen respectively into the silicon nitride structure. The amounts of aluminium and oxygen which can be substituted into this structure increase with temperature. At 1750.degree. C., z can range from 0 to 4.2. In general terms .beta.'SiAlON compositions can be referred as low z compositions and high z compositions with low z being &lt;3 and high z being .gtoreq.3. The z value basically refers to the aluminium content of the composition.
.alpha.'-phase SiAlON (.alpha.'SiAlON) has a structure derived from .alpha.Si.sub.3 N.sub.4 which is stabilised by metal cation (M) such as Y, Li, Ca. The general formula is EQU M.sub.m/v Si.sub.12-(m+n) Al.sub.m+n O.sub.n N.sub.16-n
where m and n indicate the replacement of (m+N) (Si--N) bonds by m(Al--N) and n(Al--O) bonds and v represents the valency of the metal cation M. The limits of the .alpha.SiAlON composition are restricted and vary with the size and nature of the stabilising cation. For example, the limits of solubility of yttrium have been found to vary m/v from 0.33 to 0.67 for one composition range. A limited range of metal cations stabilise the .alpha.SiAlON structure. These are Li, Ca, Mg, Y and a number of the rare earth metals but not La or Ce. SiAlONs are advanced ceramic materials which exhibit useful properties such as high strength and hardness, low density, wear resistance and corrosion resistance, and are able to retain these properties at high temperatures. .alpha.'SiAlON, when fully dense, is a very hard material but brittle. .beta.'SiAlON is less hard but tough. A composite of the two is a good compromise and yields excellent mechanical strength and wear resistance. SiAlONs are used in refractories and for a variety of engineering applications such as cutting tools, spray nozzles and pump seals. The exact properties of a given SiAlON depend on the chemical composition and fabrication variables, such as purity, grain size and shape, and the method of fabrication. .beta.'SiAlON has similar properties to silicon nitride which include excellent resistance to attack by molten metal. Silicon nitride is commonly used as a refractory material.
Documents indicating the state of the art include:
U.S. Pat. No. 3,960,581 to Ivan B Cutler discloses a process for making SiAlONs from readily available raw materials, such as clay, together with carbon. There is no teaching or recognition however of the use of silicon metal in the process. Use of silicon metal allows synthesis of low z SiAlON compositions. In addition, the use of carbon is preferable, but not essential, in the process of the present invention. It is also low z .beta.'SiAlON compositions that allow the formation of .alpha.'SiAlON compositions by the process of the present invention as will be further described herein.
DD 263749 to Akad Wissenschaft DDR, inventor Schikore H, which describes the production of SiAlON-based materials from a charge containing by weight (A) 75-95% clay, 5-25% carbon, and 0-50% aluminium compounds; or (B) 50-80% clay, 20-50% silicon carbide, and 0-50% aluminium compounds. No disclosure of the use of silicon metal is made and carbon or silicon carbide must be used.
U.S. Pat. No. 4,360,506, inventor Paris R A, which discloses the formation of .beta.'SiAlON's from a paste comprising silico-aluminous material (clay), carbon, and fine particles of a ligneous material (eg sawdust). The carbon and ligneous material are essential and no mention of the silicon metal is made.
U.S. Pat. No. 4,871,698, inventors Fishler et al, uses silicon metal in the production of a refractory body. The other constituents include carbon, .beta.'-SiAlON, clay, silica and silicon carbide amongst others.
Other common methods for producing .alpha.'SiAlON's and .beta.'SiAlON's include:
(i) Reaction Sintering of mixtures of two or more of the following: Si.sub.3 N.sub.4, SiO.sub.2, Al.sub.2 O.sub.3, AlN, and AlN-polytypoids, at .gtoreq.1600.degree. C. under a nitrogen atmosphere, usually in the presence of a rare earth sintering aid such as Y.sub.2 O.sub.3 or CeO. This process involves expensive raw materials and high temperatures, but allows good control over the composition and purity of the product. PA1 (ii) Carbothermal Reduction. Aluminosilicate minerals are blended with carbon and fired at .gtoreq.1350.degree. C. under a flowing nitrogen atmosphere. This process is described as carbothermal reduction because the carbon acts by reducing the aluminosilicate, allowing nitridation to occur. This process involves cheap raw materials and lower firing temperatures than for reaction sintering but impurities in the aluminosilicate can degrade the properties of the product. The process is more difficult to control because it involves stopping a reaction at a specific point prior to completion. PA1 (iii) Combustion Synthesis. A mixture containing silicon metal powder is ignited under a nitrogen atmosphere. The energy evolved by the strongly exothermic nitridation of silicon propagates a reaction front through the reaction mixture. This method is very rapid and energy efficient but is difficult to control. PA1 a) heating the components at a rate of about 1.5.degree. C. to about 10.degree. C. per minute, to a temperature of about 1350.degree. C. to 1900.degree. C. under a flowing N.sub.2 atmosphere having about .ltoreq.0.5% oxygen and about .ltoreq.0.5% water vapour; PA1 (b) holding the temperature between about 1350.degree. C. and about 1900.degree. C. for up to about 8 hours; and PA1 (c) recovering the formed product. PA1 (a) heating the components at a rate of about 1.5.degree. C. to about 10.degree. C. per minute, to a temperature of substantially 1350.degree. C. to 1900.degree. C. under a flowing N.sub.2 atmosphere having about .ltoreq.0.5% oxygen and about .ltoreq.0.5% water vapour; PA1 (b) holding the temperature between about 1350.degree. C. and about 1900.degree. C. for up to about 8 hours; and PA1 (c) recovering the formed product. PA1 a) heating the components at a rate of between substantially 1.degree. C. and 20.degree. C. per minute, to a temperature of substantially 1100.degree. C. to 1750.degree. C. under a flowing nitrogen or nitrogen containing atmosphere having about .ltoreq.0.5% oxygen and about .ltoreq.0.5% water vapour; PA1 b) holding the temperature between about 1100.degree. C. and about 1750.degree. C. for up to about 12 hours; and PA1 c) recovering the product.
Methods (ii) and (iii) both yield .beta.'SiAlON powders which must then be formed and sintered to obtain a ceramic body. Method (i) is the most commonly used method for preparing .alpha. and .beta.'SiAlON. As is apparent from above known methods, in order to get good control over the composition and purity of the product expensive raw materials and/or extreme reaction conditions are required.
It is an object of the invention to provide an improved process for the production of .alpha.' and .beta.'SiAlONs.